1. Field of the Invention
Generally, the present invention relates to a metal-coated carbon article. More specifically, the present invention relates to a platinum coated carbon fuel cell electrode.
2. Description of the Related Art
A fuel cell is an electrochemical device in which electrical energy is generated by a chemical reaction without altering the basic components of the fuel cell, i.e., the electrodes and the electrolyte. Fuel cells combine hydrogen and oxygen without combustion to form water, and also produce direct current electric power. The process can be described as electrolysis in reverse. The fuel cell is unique in that it converts chemical energy continuously into electrical energy without an intermediate conversion to heat energy.
Fuel cells have been pursued as a source of power for transportation because of their high-energy efficiency (unmatched by heat engine cycles), their potential for fuel flexibility, and their extremely low emissions. Fuel cells have potential for stationary and vehicular power applications. The commercial viability of fuel cells for power generation in stationary and transportation applications depends upon solving a number of manufacturing, cost, and durability problems.
The operation of a fuel cell involves supplying fuel to an anode, where the fuel is ionized to free electrons and hydrogen ions. The free electrons flow through an external circuit to the cathode. The free hydrogen ions pass through the electrolyte to the cathode, which is supplied with oxygen. The oxygen at the cathode is ionized by the electrons, which flow into the cathode from the external circuit connected to the anode. The ionized oxygen and hydrogen ions react to form water.
Fuel cells are broadly classified by operating temperature level, type of electrolyte, and type of fuel. Low-temperature fuel cells (less than 150° C./302° F.) require a catalyst in order to increase the rate of reaction to a level that is high enough for practical use. Electrodes for low temperature fuel cells usually are porous and impregnated with the catalyst.
Among the several types of fuel cells under development to provide efficient sources of electrical power with reduced pollution, cells using gas diffusion electrodes (GDEs) with proton exchange membranes as the electrolyte (proton exchange membrane fuel cells, PEMFCs) are seen as having a number of advantages. Such fuel cells avoid the problems of handling liquid fuels and electrolytes because they use gaseous reactants and a solid electrolyte that allows the transfer of protons between electrodes. Although phosphoric acid fuel cells (PFC) utilize phosphoric acid as the electrolyte, the same catalyst loaded electrodes described above are required for their functions. The cells have been found to be reliable, efficient, and convenient sources of power. However, they have proven to be very expensive in terms of cost per kilowatt of power delivered. As a consequence, their practical application has been limited to specialized applications that can justify their considerable expense, e.g., in aerospace applications. If such fuel cells are to achieve wider application, for example, as sources of power for automotive propulsion or stationary power plants, the cost in terms of dollars per delivered kilowatt will have to be significantly reduced.
Further, low temperature fuel cells cannot be used successfully for vehicular propulsion unless the fuel cells have a very large electrode area coated with a catalytically active material. The noble metal catalysts used in low temperature fuel cells generally perform most efficiently if they are in small clusters of nanometric dimensions on a suitable support. The support material must be: (a) electrically conductive; (b) chemically inert in the fuel cell environment; (c) mechanically robust; (d) sufficiently adherent to the cell membrane; and, (e) depositable in a thin film form, into which platinum, or other catalyst material, can be incorporated.
A major factor in the cost of PEMFCs and PFC is the expense of the electrodes. The cost of the electrodes is determined by a number of factors, primarily the expense of the precious metal catalysts, which are needed for practical efficiency, and the cost of fabricating the electrodes, which is typically conducted by means of a batch process. Furthermore, the cost of the fuel cell system is also greatly affected by the electrochemical performance of the electrodes, which determines the power density of the fuel cell, i.e., the power produced per unit area, e.g., kilowatts per square centimeter. The combination of power density, catalyst loading, and system fabrication costs determines the ultimate cost per kilowatt of the complete fuel cell system.
A favored material for use as an electrode support material is carbon. The carbon typically is “doped” with 1-10% platinum or platinum-ruthenium. In the past, the catalyst-doped carbon has been applied in the form of ink. Alternatively, the catalyst-doped carbon can be applied using complex chemical processes that require high temperature firing, resulting in a glassy carbon that contains platinum oxide. The high temperature firing used to produce these electrodes, cannot be used to coat the ionomer membranes that are favored for use in polymer electrolyte fuel cells (PEFC's).
Some of the most efficient electrocatalysts for- low temperature fuel cells are noble metals, such as platinum, which are very expensive. It has been estimated that the total cost of such catalysts comprises a significant percentage of the total cost of manufacturing a low-temperature fuel cell. The expense of such catalysts makes it imperative to reduce the amount, or loading, of catalyst required for the fuel cell. This requires an efficient method for applying the catalyst. The method also must produce an electrocatalytic coating with a minimal catalyst load that also has sufficient catalytic activity for commercial viability.
Conventional fuel cell electrodes have used unsupported platinum black, having a surface area of about 28 m2/g with a particle size of about 10 nanometers, at a catalyst loading of about 4 mg/cm2 of electrode area. It is estimated that the amount of precious metal will have to be reduced substantially below 1 mg/cm2 if PEMFCs are to become a widely used source of electric power.
In a typical process, an amount of a desired noble metal catalyst, in the range of about 0.5-4 mg/cm2, is applied to a fuel cell electrode as an ink or using complex chemical procedures. Unfortunately, such methods require the application of a relatively large load of noble metal catalyst in order to produce a fuel cell electrode with the desired level of electrocatalytic activity, particularly for low temperature applications. The expense of such catalysts makes it imperative to reduce the amount, or loading, of catalyst required for the fuel cell. There is therefore a need in the art for a more efficient method for applying the catalyst.
In recent years, a number of deposition methods, including rolling/spraying, solution casting/hot pressing, and electrochemical catalyzation, have been developed for the production of platinum catalyst layers for proton exchange membrane (PEM) fuel cells. Although thin sputtered platinum coatings deposited on carbon cloth can measurably improve fuel cell performance, this approach generally is not considered to be viable for large area deposition or as a stand-alone treatment for applying platinum. Continuing challenges remain in the development of scalable methods for the production of large-area (>300 cm2), high performance (>1 A/cm2 at 0.6 V) fuel cell electrodes with low platinum loadings (<0.3 mg/cm2).
Additionally, it has been recognized that the amount of precious metal catalyst can be reduced if the metal is present in a more finely divided form. Consequently, electrodes using platinum supported on a granular support, e.g., carbon particles, have been used. Such supported platinum catalysts, prepared by chemical precipitation of the metal onto the granular support, typically have surface areas of about 120 m2/g, with a particle size of about 2-2.5. nanometers, and a catalyst loading of about 0.5 mg/cm2. Although these electrodes use less of the costly platinum catalyst, the power density obtained using such electrodes has been less than satisfactory. Accordingly, the cost of such a fuel cell system is still too high. It is believed that the relatively poor performance, i.e., low power density, is caused by ineffective utilization of the catalyst, because a substantial fraction of the platinum is not accessible to the reagents.
A method for depositing precious metal catalyst in finely divided form irla gas diffusion electrode is disclosed in U.S. Pat. No. 5,084,144, to Vilambi-Reddy et al. According to the method of the '144 patent, fine particles of a catalytic metal are deposited electrolytically onto an uncatalyzed layer of carbon particles, bonded with a fluorocarbon resin, and impregnated with the proton exchange resin by contacting the face of the electrode with a plating bath and using pulsed direct current. The gas diffusion electrodes prepared by the process of the '144 patent contained about 0.05 mg/cm2 of platinum as particles of about 3.5 nanometers in diameter having a surface area of about 80 m2/g. Such electrodes functioned about as well as the electrodes using supported platinum with a loading of 0.5 mg/cm2 of platinum. It is believed that these electrodes achieved their improved mass activity, i.e., current per weight of platinum, because the electrolytic process deposits the catalyst particles only at regions with both electronic and ionic accessibility. Such locations are expected to be accessible to the protons and electrons required for the fuel cell reactions. However, such improved mass activity does not compensate for the low catalyst loading provided by the process disclosed in the '144 patent. Consequently, the power density of such electrodes is still insufficient to permit the wide use of PEMFCs as sources of electric power.
It would, therefore, be useful to develop a method of depositing catalytic metals on carbon articles in amounts greater than hitherto achieved, while retaining the small particle size, and electronic and ionic accessibility that provides high mass activity.